Both CSF and serum levels of C3a, C5a, and the soluble terminal complement complex SC5b-9, indicative for general activation of the complement system, of C4a, specific for the activation of the classical pathway, Ba and Bb, reflective for alternative complement activation as well as concentrations of complement-inhibitory proteins factor H and factor I were unchanged in patients with PACNS as compared to patients with NIND (Figure 1). The gene discussed is C3; the disease is primary central nervous system vasculitis.